1. Field of the Invention
The invention concerns a fluid filter element and method for forming same.
2. Description of Prior Art
Fluid filter elements are well known and are widely used in fluid filters for varying areas of use, including hydrodynamic machines and air-conditioning engineering. Their meander-shaped or zig-zag structure provides a substantially increased filter area, in relation to a fixed afflux flow cross-section. In order to fix the fold walls of such a structure relative to each and to support them with respect to each other, various different kinds of design configurations have been developed. Filters with depressions which are embossed into the flat filter material and which, when the material is folded, bear against each other and support each other are known.
In the case of earlier fluid filter elements of that kind the embossings of the fold walls were simply laid against each other when executing the folding operation and the filter element which was finished as a kind of xe2x80x9cfold packxe2x80x9d was externally fixed by being clamped in a box or frame. While such particularly simple fluid filter elements certainly operate satisfactorily at the beginning of their period of use and in areas of use without involving fluctuating pressure loadings worth mentioning, critical stability problems are found to occur in relation to certain degrees of filling with deposited dust particles, or dust loading, and in particular when used in relation to fluid-dynamic machines.
A relatively long time ago, therefore, the inventor proposed a filter structure in which the mutually facing embossings of the filter material are glued to each other, in the folded condition, on their respective top sides. The gluing operation is implemented by a selective thin application of adhesive or suitable impregnation of the filter material in the region of the embossings. Such a filter structure is described for example in U.S. Pat. No. 5,290,447.
In connection with rising demands on the part of users in terms of performance and service life of the fluid filter elements, there was a trend for an increase in the fold height, which at the same time results in an increase in the maximum spacing of the filter material surfaces from each other. In that respect, difficulties arose in regard to implementing those increasing spaces solely by increasing the depth of the embossing regions which support the fold walls relative to each. In particular, with embossings which are of an increasing depth, there is an increased risk of through-embossings or piercings, holes or at least very thin areas in the filter material. Fluid filter inserts with such piercings are rejects.
Therefore, in a further stage in development, the proposal was made that, instead of an application of adhesive without a thickness worth mentioning on the top sides of the embossings, adhesive threads of considerable thickness should be applied, which together with the embossings afford the required maximum spacing between the fold walls from the filter material. That structure is described for example in U.S. Pat. No. 5,804,014.
In a next step in developmentxe2x80x94the inventor proposed to use a structure which is controlled variably in respect of its height (specifically rising linearly towards the opening of the folds), in particular also entirely without any embossing or at any event without embossing which is variable in respect of its depth, in the filter material. This therefore involves fluid filter elements in which the spacing function and the supporting function as between the fold walls of a zig-zag fold configuration is achieved substantially solely by adhesive threads which are variable in height. That structure is described for example in DE 197 55 466 A1.
In the above-mentioned fluid filter structures, depressions and raised portions are embossed into the filter material alternately in the widthwise direction of the filter material and in a direction perpendicularly to the fluid flow direction and parallel to the material surface, so that (in the folded condition of use) support regions are disposed alternately at both sides of the filter medium. In a situation involving long-term use of those filter structures under high levels of loading, in particular in fluid filters for fluid dynamic machines which entail a pronounced fluctuating pressure loading, and also in the case of filters for suspended matter or mechanical filters which can be cleaned off, problems arose with reliability.
In particular, it is observed that the filter medium in such devices can be torn away from the adhesive threads under a high loading. The filter medium can also be delaminated in a layer-wise fashion. The reason for this is that the boundary layer between the adhesive thread and filter medium is subjected to a heavy tensile loading on the feed air or upstream side. In the regions where the filter medium has come away from the adhesive thread, it begins to xe2x80x9cflutterxe2x80x9d. The entire filter element then becomes unstable from the fluid dynamics point of view, the folds on the discharge air or downstream side collapse, and enormous pressure difference rises occur which can reach a level practically involving local air impermeability of the filter. As the ultimate effect, the filter is destroyed.
Therefore, in an unpublished German patent application, the inventor proposed a specially reinforced structure for high-efficiency fluid filters of that kind. It is provided in that structure that, in all embossing regions, the filter material fold walls are not only glued to each other on the mutually facing raised portions, but in addition a particularly high adhesive thread is provided in lateral orientation with those adhesive meansxe2x80x94that is to say in the depressions on the rearward surface of the filter material, which depressions correspond to the raised portions. In that structure the filter material is therefore xe2x80x9cclamped in positionxe2x80x9d on both sides between adhesive threads in all gluing regions.
A technological problem of this last-mentioned arrangement however is that the adhesive threads must be formed of very great height, in the depressions in the filter material. When using conventional hot melt adhesives and manufacturing installations, relatively wide adhesive threads are also formed in that situation, which detrimentally reduce the usable filter area. In particular the amounts of adhesive become so great that, in the case of the adhesive threads which are to be applied on the underside of the web of filter material, the hot melt adhesive drips out before it hardens. The usual application of adhesive to the horizontal disposed filter material, which is advantageous from the point of view of the apparatus configuration, is thus no longer possible. In addition, the application of controlled adhesive threads to both sides of a filter medium is in any case technologically demanding and presupposes the use of metering devices which operate precisely and which are correspondingly expensive.
FIGS. 1-6 further illustrate various prior art structures and problems.
Referring to FIG. 1, shown therein is an isometric view of a part of a filter material 10 in a lying condition, for the production of a fluid filter element in accordance with the prior art; embossed into the filter material 10 on both sides of the plane of the material are depressions and raised portions respectively as indicated at 11, which are approximately triangular in longitudinal section, that is, they extend longitudinally at an angle with respect to the plane of the material, and between approximately trapezoidal and rectangular in cross-section, typically in a soft curve, which extends in the longitudinal direction of the filter material web. Reference numeral 13 denotes fold edges which extend perpendicularly thereto and which are also embossed into the filter material. The embossings 11 extend virtually over the entire spacing xe2x80x9caxe2x80x9d between adjacent fold edges 13 and embossing height is illustrated at xe2x80x9cb1xe2x80x9d.
FIG. 2 is a diagrammatic isometric view of a part of a prior art fluid filter element 20 which is produced by folding a pre-treated filter material of the kind shown in FIG. 1. The embossings 21 are somewhat more rounded in the longitudinal direction than the embossings 11 shown in FIG. 1. The folding procedure results in a filter element with a fold height xe2x80x9caxe2x80x2xe2x80x9d which, by virtue of the wedge angle of the V-shaped folds, is slightly smaller than the spacing xe2x80x9caxe2x80x9d of the pre-embossed fold edges as shown on FIG. 1. The maximum fold spacing xe2x80x9cc1xe2x80x9d between laterally adjacent fold edges 23 is somewhat greater than double the embossing height xe2x80x9cb1xe2x80x9d. Since, as can be clearly seen from FIG. 2, the embossings 21 bear directly against each other with their top sides even in an adhesively joined structure, the spacing between adjacent fold walls 25 is afforded solely by the embossings 21.
FIG. 3 is a diagrammatic view in cross-section showing the structural principle of a further known fluid filter element 30. In this case also a filter medium 30xe2x80x2 is provided with embossings 31 which project alternately towards both sides. In this case the embossings are of a semicircular configuration in cross-section. The material is laid in folds along pre-embossed fold lines 33a, 33b (in which respect those references distinguish upper and lower edges of the folds of the folded configuration formed from each other).
In this case however, prior to the folding operation, relatively thick adhesive threads 37 which are also approximately semicircular in cross-section are applied. In the folding operation, while in the condition of not yet having hardened, the adhesive threads 37 come into contact with each other and are joined to each other. The fold walls 35 made up of the filter material 30xe2x80x2 are also connected together by way of the adhesive threads, more specifically with a fold spacing xe2x80x9cc2xe2x80x9d which is substantially greater than double the embossing depth xe2x80x9cb2xe2x80x9d. Arrows A1 and A2 respectively indicate into which fold openings feed or upstream air flows (A1) and out of which openings exhaust or downstream air is discharged (A2).
FIGS. 4a through 4c illustrate in a sketch fashion the loading characteristic of a prior art fluid filter element of this kind, wherein (in spite of the slightly differing geometry of the arrangements), the reference numerals from FIG. 3 have been adopted. FIG. 4a shows a prior art xe2x80x9cvirginxe2x80x9d filter geometry at the beginning of the period of use, FIG. 4b (prior art) shows a condition with the beginning of fold deformation after the fold walls which in the feed air region are subjected to a high tensile loading have torn away from the adhesive threads, and FIG. 4c shows the condition involving major or continual loading in which the fold deformation is already far advanced and is irreversible and such as to have an adverse effect on function.
FIGS. 5a and 5b diagrammatically show two mutually similar fluid filter elements 50A and 50B, in which additional stabilization is provided to prevent the detachment and deformation phenomena which are diagrammatically illustrated in FIGS. 4b and 4c. The references used in these Figures are based on those employed in FIG. 3 so that there is no need for further description in this respect. The shape of the embossings 51 and the adhesive threads 57 also corresponds to the embodiment shown in FIG. 3.
The essential difference lies in the provision of additional adhesive threads 58 on the underside of the raised embossings 51, that is to say in the depression respectively corresponding to a raised portion. It can be clearly seen from both Figures that naturally those adhesive threads 58 must be of a much greater height than the adhesive threads 57 arranged on the raised portions, in order to be able to bridge over the spacing between the fold walls 55 (that spacing being much greater in the recesses), when they are brought into contact with each other in the folding operation. The difference between the filter elements 50A and 50B is that in the case of the former the additional adhesive threads 58 are provided only in every second adhesive region, while in the latter they are provided in all adhesive regions.
FIG. 6 is once again a view in longitudinal section showing an embodiment of a known filter design principle which is diagrammatically illustrated in a cross-sectional view in FIGS. 5a and 5b. On the raised side of the embossings 61 of the filter walls 65, a fluid filter element 60 of this type has adhesive threads 67 while between the corresponding depressions it has adhesive threads 68 of substantially greater height. Both the flatter adhesive threads 67 and also the higher adhesive threads 68 here extend with bridging regions 67a and 68a respectively over the respective outer fold edges 63a and 63b. 
The object of the present invention comprises providing a fluid filter element of a structure which is substantially simplified in regard to the adhesive connections, in which the above-mentioned problems do not occur and which nonetheless can withstand high mechanical loadings and which satisfies high demands in terms of reliability and operating life.
The foregoing object is attained by a filter element comprising an embossed flat filter material which is folded so as to define a plurality of substantially adjacent walls, each wall having an embossing comprising a first embossing portion which extends from one side of said wall and a second embossing portion which is adjacent to said first embossing portion and which extends from the other side of said wall, and an adhesive connecting adjacent embossings of adjacent walls. The adhesive preferably has a substantially constant height.
The invention combines two essential concepts: 1. The concept of moving away from an application of adhesive to the filter medium, with such application of adhesive being controlled variably in respect of height; and 2. The concept of a positive-negative embossing of the filter medium which is combined in one and the same embossing region. The implementation of the first concept is possible by virtue of execution of the embossing in accordance with the second concept without cutbacks in regard to the functionality of the filter element. It is possible in that way to achieve substantial advantages over the previous filter element designs and production technologies as follows:
(1) a saving of adhesive in comparison with the previous filter element with xe2x80x9csupport threadsxe2x80x9d;
(2) a symmetrical fold-adhesive-thread-geometry, with uniform clamping on both sides and support for the filter medium between adhesive threads;
(3) in comparison with the previous filter designs without xe2x80x9csupport threadsxe2x80x9d, the invention makes it practically impossible for the filter medium to tear away from the adhesive or for the filter fleece or non-woven cloth to delaminate; and
(4) a considerable degree of simplification in the installation of components, their installation and operation for adhesive application, including simplified control and maintenance, and so forth.
A particular advantage of the filter element of the present invention is that laterally mutually aligned adhesive threads are arranged both on the top side and also on the underside of each fold wall so that the filter material is clamped in each gluing region on both sides between adhesive threads. The forces which act on the filter material on the upstream flow side are thereby reliably withstood and supported and detachment of the filter fleece from the adhesive or delamination of the filter fleece is practically excluded.
Due to this heightened structural integrity, the distance between adhesive threads can be increased which in turn blinds off less filter material which in turn renders a filter with more available/working filter material, higher performance and longer life.
In a further preferred embodiment of the invention, the first and second portions of each embossing, which are mutually adjoining in the fluid flow direction, are at least approximately axially symmetric in relation to an axis of symmetry which extends in the center between the fold edges on the fold wall perpendicularly to the fluid flow direction. The shape of the first and second embossing portions is at least approximately triangular (in a section plane parallel to the fluid flow direction).
The apex of the configuration which is shaped out of the fold wall (in the first embossing region) and the bottom of the recess or part which is shaped into the fold wall (in the second embossing portion) are preferably disposed in one and the same plane and that plane is advantageously substantially parallel to the fluid flow direction and to other embossings. This makes it possible to adhesively join mutually adjacent fold walls using adhesive threads, spots or dots of precisely constant height, that is to say it permits the use of adhesive-applicator devices with a constant adhesive discharge amount per unit of time or per unit of length with a continuously traveling filter material.
In order to optimize the adhesive thread pattern on the finished filter element or fold pack, an additional adhesive spot or short adhesive thread can be added to the adhesive threads which moreover are produced with a constant adhesive discharge amount per unit of time or per unit of length of filter material, in the immediate area around the later outer fold edges. That permits adhesive threads to be caused to extend over the outer fold edges and thus provides a further increase in the stiffness and rigidity of the fluid filter element.
In a further embodiment, a short portion is cut out in the otherwise continuous adhesive threads in the immediate area around a later inner fold edge or a xe2x80x9cfold bottomxe2x80x9d. That prevents the adhesive from being pressed wider at the inner edge when folding the filter material, and thus avoids the surface being unnecessarily greatly covered with adhesive.
If desired, for example to save adhesive, the application of adhesive may also be effected intermittently or in spot form over the entire length of the fold walls. In that way, the surface area which is covered with adhesive and which thus can no longer be used as a filter area is reduced, which has a positive influence on the filtration properties such as degree of separation, pressure difference, and dust holding capability of the filter. It will be appreciated, of course, that the mechanical stiffness and rigidity of the assembly is somewhat reduced as compared to a filter with continuous adhesive threads.
Besides the positive-negative main embossings which carry the adhesive threads or spots, further embossings which are not provided with adhesive can be provided in the otherwise flat region of the filter material. Those additional embossings can be shaped in a particularly simple fashion for example in the form of continuous grooving. Those additional embossings have the following effects:
(1) the free filter area is increased, that is to say a gain in the area of the filter medium is achieved, which results in a reduction in pressure difference or an increase in dust holding capability;
(2) the filter medium is additionally stabilized in the machine-running direction, which is helpful in particular in the operation of folding high folds;
(3) any instability in the filter medium at the fold center (that is to say in the region of the above-mentioned axis of symmetry or transition of the main embossings) is avoided, which also makes the folding operation easier; and
(4) in the situation involving a high loading, total collapse (folding together) of adjacent fold walls in the region between the glued main embossings is avoided. The total amount of adhesive between two filter surfaces inside a fold, in a preferred embodiment, can comprise two portions or components which are applied to the respectively adjoining material surfaces and which are connected together when the filter element is folded together. In this manner, the partial threads or partial spots are applied having substantially the same volume so that the application of adhesive to the filter material, which passes through beneath the applicator devices in a continuous operation, with a constant adhesive discharge amount per unit of time, can be implemented continuously substantially over the length of the web. This is advantageous in that when adhesive is applied, it is applied at a constant volume or discharge rate from the adhesive nozzles to the filter paper, for example with simple xe2x80x9conxe2x80x9d-xe2x80x9coffxe2x80x9d commands.
In an alternative embodiment, adhesive can be applied only to one of the two filter material portions or surfaces of a fold which are to be glued together, while the application of adhesive is interrupted when the second region or surface is passing through. In this case also it is possible to operate with a constant adhesive discharge amount. However, the adhesive can only penetrate into one of the two filter fleece surfaces before the filter is folded together.
The proposed structure can advantageously be used in relation to modern fluid filter materials which are produced on the basis of glass fiber or plastic fiber or which contain both types of fiber or any other type of fiber in combination with each other. The adhesive used is a hot melt or hot melt foam, which is known per se, or any other appropriate adhesive which can be applied in the form of an adhesive spot or thread which is relatively high but nonetheless stable.